Play a sound, then another when the first is finished?
Here's what I ended up doing: there's a property time that I did not know about: Timer. I then set a property sensor to track this "Timer" value, but the "equal" eval type did not work, I had to use "interval" and set my interval using the right n.000000 notation. By precisely tuning the Minimum value, I managed to get a sample-perfect intro+loop sequence.